In order to assess late results with the Hancocl bioprosthesis, 111 patients undergoing matral valve replacement (MVR) either alone (56) or in conjunction with another type of prosthetic valve, prior to 1975 were evaluated. Hospital mortality was 9.9%. Cumulative follow up is 505 patient-years (pt-yrs); mean 5.4 years. Late mortality for isolated MVR is 4.3+-1.3% pt-yrs; actuarial survival is 82+-6% at five years and 65+-11% at ten years. Anticoagulation therapy was employed only for documented emboli or concomitant mechanical aortic prostheses. The incidence of emboli was .3.+-0.9% pt-years for 62 patients with bioprostheses only. Emboli occured in 22% of patients atrial fibrillation compared to 3.8% of patients in normal sinus rhythm. Intrinsic mitral bioprosthesis failure occurred in ten patients; two died. Actuarial late survival, free of intrinsic mitral bioprosthetic failure was 99+-1% at five years, 92+-4% at seven years, 70+-12% at nine years, and 61+-13% at ten years. Early hemodynamic function continues to be good, although 7 of 18 patients studied late (avg 85 mos) were followed for possible late deterioration. Intrinsic mitral valve destruction secondary to infection has been observed in only one patient. The question remains whether the long-term risk of late intrinsic valve failure and reoperation will outweight the low incidence of emboli and avoidance of anticoagulant related hemorrhage